Production, refining and recycling of lightweight and reactive metals in ionic liquids

ABSTRACT

Lightweight and reactive metals can be produced from ore, refined from alloy, and recycled from metal matrix composites using electrolysis in electrolytes including an ionic liquid containing a metal chloride at or near room temperature. Low electric energy consumption and pollutant emission, easy operation and low production costs are achieved.

[0001] The U.S. Government has a paid-up license in this invention andthe right in limited circumstances to require the patent owner tolicense others on reasonable terms as provided for by the terms of GrantNo. NSF-EPS-9977239 awarded by the National Science Foundation.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to the production and purification ofmetals. In particular, this invention relates to production (extraction)from ore, refining from metal alloy, and recycling from metal matrixcomposites, of lightweight and reactive metallic elements usingelectrolysis in electrolytes at or near room temperature.

[0004] 2. Description of the Related Art

[0005] Conventional production, refining and recycling of lightweightand reactive metallic elements involves high temperature electrolysis inmolten salts. Aluminum provides an illustrative example.

[0006] For the past century, primary aluminum has been produced usingthe Bayer-Hall-Heroult process, which involves high temperatureelectrolysis of alumina dissolved in molten cryolite (Na₃AlF₆). Currentaluminum refining also uses high temperature electrolysis.

[0007] However, conventional high temperature electrolysis processesrequire the use of many expensive refractory materials. In addition,conventional high temperature processes consume large amounts of energy.Current high temperature refining of primary and recycled aluminum usesthree-layer electrolysis with even higher energy consumption thanprimary aluminum production. High temperature electrolysis processesalso produce large amounts of pollutants. CF₄ gas formed in currentindustrial processes has an extremely long atmospheric residence timeand a very high “global warming potential” (about 5100 times higher thanCO₂). Thus, high temperature electrolysis processes have severaldisadvantages, including the use of expensive refractory and electrodematerials, high production costs and high pollutant emission.

[0008] To overcome these disadvantages, electrodeposition processes ator near room temperature have been explored.

[0009] U.S. Pat. Nos. 2,446,331; 2,446,349 and 2,446,350 disclose roomtemperature electrodeposition of aluminum from a molten electrolyteconsisting of an aluminum salt, such as aluminum chloride, and an ionicliquid of a N-alkyl pyridinium halide, such as ethyl pyridiniumchloride. However, the ethyl pyridinium chloride has the disadvantage ofdissolving deposited aluminum and decreasing current efficiency.

[0010] U.S. Pat. Nos. 4,624,753; 4,624,754; and 4,624,755 also discloseroom temperature electrodeposition of metals, using as ionic liquidsnon-aqueous nitrate-amide melts to electrodeposit metals such as Fe, Ni,Zn, Ag, Pb and Cu. However, the “electrochemical window” (i.e.,difference between the lower and upper voltage limits forelectrodeposition) of the nitrate-amide ionic liquids is not high enoughfor the electrodeposition of aluminum.

[0011] U.S. Pat. No. 5,552,241 discloses low temperature molten saltcompositions comprised of a mixture of a metal halide, such as aluminumtrichloride, and fluoropyrazolium salt.

[0012] U.S. Pat. No. 5,731,101 discloses a low temperature moltencomposition comprising a mixture of a metal halide and analkyl-containing amine hydrohalide salt.

[0013] U.S. Pat. No. 5,855,809 discloses electrolytes, which do notcrystallize at ambient temperature, formed by the reaction of a strongLewis acid, such as AlCl₃, with an inorganic halide-donating molecule.

[0014] Although room temperature production and refining processes havemet with some success, the recycling of aluminum from composites ofrefractory particles in a matrix of aluminum alloy is much moredifficult technologically. Aluminum alloy composites are findingincreasing application. For example, discontinuously reinforced aluminum(“DRA”) composites are finding extensive use in the automotive industry.However, when DRA is re-melted along with regular aluminum alloy, therecycled product is very difficult to be fabricate because of the veryhigh hardness resulting from the presence of refractory reinforcedparticles, such as silicon carbide. To separate the refractory particlesfrom the aluminum alloy, it is common to filter the particles from amelt of the recycled product. However, high temperature filtrationprocesses are very difficult to perform.

[0015] Because of the difficulties encountered in conventionalprocesses, there is a need for improved methods of producing, refiningand recycling lightweight and reactive metallic elements.

SUMMARY OF THE INVENTION

[0016] The present invention provides methods and an apparatus forproducing and/or purifying a lightweight and reactive metallic elementby electrolysis at or near room temperature using an electrolyteincluding an ionic liquid containing a chloride of the metallic element.

[0017] To produce (extract) the metallic element from ore, the ore isfirst reacted with chlorine to form a gas of a chloride of the metallicelement. After purification, the chloride of the metallic element ismixed with an ionic liquid to form an electrolyte that is a liquid at ornear room temperature. Electrolysis using the electrolyte deposits themetallic element on a cathode. Chlorine gas released during theelectrolysis is recycled to react with more ore.

[0018] To purify a metallic element found in metal alloy and metalmatrix composite, an anode is first formed from the alloy or composite.The anode is placed, along with a cathode, in an electrolyte of an ionicliquid containing a chloride of the metallic element. Electrolysis at ornear room temperature dissolves the metallic element from the anode intothe electrolyte and deposits the metallic element on the cathode. Nochlorine is released. Impurity atoms and refractory particles remain atthe anode or fall to the bottom of the electrolyte.

[0019] An ionic liquid made of 1-butyl-3-methylimidazolium chloride andmetal chloride has been found to be particularly suitable inelectrolytes for electrodeposition at or near room temperature oflightweight and reactive metallic elements.

[0020] An apparatus for refining and recycling a metallic elementincludes an electrolysis cell with a packed bed cathode including aporous metal basket filled with conductive particles. The large cathodicsurface area provided by the conductive particles speedselectrodeposition of the metallic element.

[0021] The present invention provides advantages over current metalproduction processes in terms of reduced cost and pollutant emission. Inthe production of Al, for example, the present invention can reduceelectrical energy costs about 35%, prevent some gaseous pollutantemissions (e.g. CO and CF₄), and reduce solid wastes (e.g. aluminumdross).

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is a flow chart showing a metal production (extraction)process.

[0023]FIG. 2 is a flow chart showing a metal purification process.

[0024]FIG. 3 shows a metal purification apparatus.

[0025]FIG. 4 shows Al extracted from ore after being electrodeposited ona copper cathode.

[0026]FIG. 5 is an X-ray diffraction pattern of a copper cathode coveredwith electrodeposited Al extracted from ore.

[0027]FIG. 6 shows electrodes after electrorefining (cathode=A, anode=B)

[0028]FIG. 7 shows X-ray diffraction patterns of (a) an Al anode beforeelectrorefining, (b) anode residue after refining and (c)electrodeposited Al.

[0029]FIG. 8 shows optical micrographs of an anode after electrolysis.

[0030]FIG. 9 shows optical micrographs of an anode after electrolysis.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] According to the present invention, a lightweight and reactivemetallic element can be produced from an ore containing the metallicelement. The metallic element can also be purified by refining themetallic element from metal alloy containing the metallic element, andby recycling the metallic element from metal matrix composite containingrefractory particles dispersed in a matrix of the metallic element.

[0032] The metallic elements that can be produced or purified accordingto the present invention include Li, Mg, Al, Ti, Zr and Nd. Preferably,the metallic element is Al.

[0033] The production and purification processes involve electrolysis ator near room temperature. As is well known in the art, electrolysisinvolves application of a voltage across an anode and a cathode insertedinto an electrolyte. In the present invention, the voltage is in a rangeof 0 to 4 volts, preferably 1 to 3.5 volts. The electrolyte is at atemperature from 0° C. to 200° C., preferably from 25° C. to 150° C. Theelectrolyte is an ionic liquid containing a chloride of the metallicelement being produced or purified. The molar ratio of the chloride ofthe metallic element to the other ionic liquid component (e.g., otherchloride) is in a range from 1 to 2, preferably 1.3 to 1.7. The ionicliquid is an organic chloride salt. Preferably, the ionic liquid is madeby mixing AlCl₃ and 1-butyl-3-methylimidazolium chloride (“BmimCl”),BmimCl is given by the following formula:

[0034] Metal Production from Ore

[0035]FIG. 1 illustrates an embodiment of the present invention in whicha metallic element is produced (extracted) from ore. Ore 10 undergoesdehydration 12 to form a dry ore 14. The dry ore 14 is reacted withcarbon 18 and recycled chlorine gas product 38 in a chlorination process16 to form a chloride of the metallic element as gaseous product 22.This can be accomplished by, e.g., passing recycled chlorine gas product38 through a fluidized bed containing the carbon 18 and the dry ore 14.To ensure that the chloride of the metallic element is in gaseous form,it may be necessary to also provide heat (not shown) to chlorinationprocess 16. Solids 20 remaining after chlorination process 16 areremoved for disposal. Gaseous product 22 undergoescondensation/separation process 24 in which impurities 26 are separatedleaving primary chloride 28. Primary chloride 28 can be further purifiedin chloride purification process 30, which separates impurity 32 frompure chloride 34. An example of chloride purification process 30 duringextraction of Al from bauxite ore includes the removal of FeCl₃.Impurities 26 and 32 may require further treatment before release to theenvironment. Electrolysis 36 of an electrolyte including an ionic liquidcontaining pure chloride 34 electrodeposits metallic element 40 on acathode and releases a chlorine gas product as chlorine 38, which isrecycled to chlorination process 16.

[0036] Preferably the ore 10 is bauxite and the metallic element to beextracted from the bauxite is Al. In this embodiment, chlorinationprocess 16 can be described by the equation

2Al₂O₃ (in bauxite)+3C+6Cl₂→4AlCl₃+3CO₂.

[0037] In this embodiment, the gaseous product 22 is AlCl₃. BecauseAlCl₃ readily sublimes at 178° C., large amounts of heat will not haveto be added to the chlorination process 16 to form the gaseous product22. The subsequent electrolysis process 36 can be described by theequation

4AlCl₃→4Al+6Cl₂.

[0038] Thus, chlorination process 16 can consume all of the chlorine gasproduct released in electrolysis process 36. In practice some chlorinegas product may be lost during recycle, and additional chlorine gas, notshown in FIG. 1, may be necessary to replace the lost chlorine gasproduct. However, preferably more than 80% of the chlorine gas productreleased during electrolysis process 36 is recycled back to chlorinationprocess 16.

[0039] Metal Purification (Refining and Recycling)

[0040]FIG. 2 shows an embodiment of the present invention in which animpure starting material 50 first undergoes anode formation 52 to forman impure anode 54. The impure starting material 50 is electricallyconductive and contains a metallic element to be purified. In an metalrefining process, the impure starting material 50 can be an alloy of themetallic element. In a metal recycling process, the impure startingmaterial 50 can be a metal matrix composite of refractory particlesdispersed in a matrix of the metallic element. The refractory particlescan be ceramic particles. Preferably, the refactory particles areborides, such as TiB₂; carbides, such as SiC; nitrides, such as Si₃N₄and AlN; or oxides, such as Al₂O₃, of metallic and/or non-metallicelements. Electrolysis 60 of the impure anode 54 in an electrolysis cellcontaining an ionic liquid that includes a chloride of the metallicelement results in electrodeposition of pure metallic element 70 on aseed cathode 56. The pure metallic element 70 and seed cathode 56product can be melted and cast in a further process (not shown).

[0041] Anodic residue 62, which includes both undissolved anode materialand any precipitate on the bottom of the electrolysis cell, is removedfor further treatment. The undissolved anode material can be re-melted(not shown) to form another anode. The precipitate can be furtherprocessed (not shown) to recover metals other than metallic element 70and other valuable materials.

[0042] After prolonged electrolysis, dissolved impurities can accumulatein the electrolyte and lead to a decrease in the purity of theelectrodeposited metallic element 70. To avoid this decrease in purity,electrolyte 64 undergoes purification 66, and the resulting purifiedelectrolyte 58 is recycled to the electrolysis cell.

[0043] Apparatus for Metal Purification

[0044]FIG. 3 shows an metal purification apparatus according to thepresent invention that can be used for refining and recyclinglightweight and reactive metallic elements. The metal purificationapparatus serves as an electrolysis cell. Container 80 is anelectrically insulating material, such as a polymer or a ceramic, thatcan withstand temperatures of at least 200° C. Container 80 supportsimpure anode 82, electrolyte 84 and a packed bed cathode. In refiningprocesses, impure anode 82 includes an metal alloy containing a metallicelement being purified. In recycling processes, impure anode 82 includesa metal matrix composite containing the metallic element being purified.Electrolyte 84 is an ionic liquid containing a chloride of the metallicelement to be extracted or purified. The packed bed cathode fits insidethe anode 82. The packed bed cathode includes cathode lead 86 for makingelectrical contact to porous basket 88 containing electricallyconductive particles 90. Cathode lead 86 and porous basket 88 are metal,and are preferably formed of the metal being electrodeposited, e.g., Al.Alternatively, cathode lead 86 and porous basket 88 can be formed ofstainless steel or copper. The conductive particles 90 are in electricalcontact with the cathode lead 86 and porous basket 88. Porous basket 88is a mesh or perforated sheet porous enough to allow electrolyte 84 anddissolved material from anode 82 to circulate in the electrolysis cell,thus improving mass transport and preventing concentration polarization.Conductive particles 90 can be of any electrically conductive material,e.g., carbon, and are sufficiently large not to slip through porousbasket 88. Preferably, the conductive particles 90 do not form a thickinsulating oxide when exposed to air. Because the conductive particlespresent more surface area than a traditional plate cathode, the packedbed cathode of the present invention is particularly suited for slowelectrodeposition processes and for dilute electrolyte electrolysis.

[0045] After electrodeposition of a metallic element on the packed bedcathode, the entire packed cathode can be lifted out of the electrolysiscell. The electrodeposited metallic element can then be separated, ifdesired, from the conductive particles 90, porous basket 88 and cathodelead 86.

EXAMPLES Example 1 Production of Al from Alumina

[0046] To extract aluminum from alumina, a mixture of alumina powder andgraphite powder is first prepared. The mixture is heated above 200° C.Cl₂ gas is passed through a fluidized bed of the hot mixture producingAlCl₃ and CO₂ gases. The AlCl₃ gas is condensed and purified. Anelectrolyte containing the purified AlCl₃ and BmimCl in a molar ratio ofAlCl₃ to BmimCl of 1.5 is prepared. A copper anode and a copper cathodeare introduced into the electrolyte. The electrolyte is maintained at atemperature of 105° C. Application of a voltage of 3.0-3.4 V across theanode and cathode causes Al to electrodeposit on the cathode. FIG. 4shows the Al deposited on the copper cathode. FIG. 5 shows an X-raydiffraction pattern of the copper cathode with the Al deposit.

[0047] Current industrial production of Al is by electrolysis of aluminadissolved in molten cryolite (Na₃AlF₆). The electrolytic cells operateat around 1000° C. Table I compares typical experimental conditions forelectrodeposition of Al according to the present invention with typicalconditions found in current industrial processes. In contrast to the1000° C. temperatures necessary in the industrial processes, the presentinvention can electrodeposit Al at 105° C. In addition, while typicalindustrial processes emit sizeable amounts of CO and CF₄ pollutants, thepresent invention produces no CO or CF₄. TABLE 1 Comparison of typicalexperimental conditions for extracting Al from alumina ElectrolysisElectrolysis in present in current Parameters invention industrialpractice Cell voltage, V 3.0-3.4 4.2-5.0 Energy consumption, Kwh/lb4.3-4.8 6.0-8.5 Current density, A/m² 400-800 — Electrode distance, mm20 100 Temperature, ° C. 105 1000 Electrode area, cm² 2 — Al depositionthickness, mm 0.1-0.2 — CO emission, kg/ton-Al 0 340 CF₄ emission,kg/ton-Al 0 1.5-2.5

Example 2 Purification of Al Alloy

[0048] An anode was prepared for electrorefining. The anode had thecomposition shown in Table 2. TABLE 2 Composition of impure aluminumanode Element Atomic % Al 79.77 Si 11.62 Fe 0.758 Cu 5.00 Mn 0.187 Mg0.0619 Cr 0.0461 Ni 0.0784 Zn 2.32 Pb 0.0713

[0049] The anode and a copper cathode were weighed and positioned about2 cm apart in an 50 ml beaker on a hot plate stirrer. An electrolyte ofanhydrous AlCl₃ and BmimCl was weighed and mixed in the beaker under aninert atmosphere to avoid moisture. The molar ratio of anhydrous AlCl₃to BmimCl was 1.5. Stirring and heating rates were set. When electrolytetemperature was stable at 105° C., a constant cell voltage of 1.0-1.5volts was applied between anode and cathode, and Al was electrodepositedat the cathode. After the electrolysis, the anode and cathode are takenout of the cell, washed with water and weighed again to measure therespective loss and gain. The anode and cathode were characterized usinga micro image analyzer, X-ray diffraction and an optical microscope.

[0050]FIG. 6 shows the cathode (A) and anode (B) after the electrolysis.Cathode A includes an Al deposit. The aluminum deposit on the cathode Acan be divided into two parts: a planar layer adjacent to the cathode Aand a dendritic layer on the planar layer. Anode B includes a dark anoderesidue where the anode B was in the electrolyte and waselectrochemically dissolved. The anode residue has a porous structureand was scratched for characterization.

[0051]FIG. 7 shows X-ray diffraction patterns taken of (a) the originalaluminum anode B, (b) the dark anode residue and (c) theelectrodeposited aluminum, after separation from cathode A. Based onX-ray diffraction patterns and spectrometer results, the original anodeB contained mainly Al, Si, Cu, Zn and Fe, as shown in X-ray diffractionpattern (a) of FIG. 7. During the electrolysis, the dissolution of eachof Al, Si, Cu, Zn and Fe should be dependent on the electrode potentialand kinetic conditions. Because Al has a lower potential than Si, Cu, Znand Fe, thermodynamically Al should be dissolved before the Si, Cu, Znand Fe. Confirming the dissolution of Al from the anode B before Si, Cu,Zn and Fe, X-ray diffraction pattern (b) of the anode residue includesthe diffraction peaks of Si, Cu, Zn and Fe, but not Al. In addition,X-ray diffraction pattern (c) of the cathode deposit includes only thepeaks associated with Al, showing that Si, Cu, Zn and Fe were notdeposited on the cathode. The Si, Cu, Zn and Fe should remain on theanode B or precipitate to the bottom of the electrolysis cell.

[0052]FIGS. 8-9 are optical micrographs of the anode B before and afterelectrolysis, respectively. The lighter area on each micrographcorresponds to aluminum. The darker area on each micrograph correspondsto impurities. The composition of the different regions of the anode Bwas determined using an optical microscope. Point B of FIG. 9 shows thatimpurities did not dissolve during in the electrolysis.

Example 3 Electrolyte Containing 1-butyl-3-methylimidazolium chloride

[0053] Electrolytes were made by mixing 0.17 moles of AlCl₃ and 0.11moles of 1-butyl-3-methylimidazolium chloride. An anode and a cathodewere placed in the electrolyte. The electrolyte was heated to 100° C. Avoltage of 1.5 volts was applied across the anode and cathode.Remarkably, electrodeposition of Al was only observed using a1-butyl-3-methylimidazolium chloride-AlCl₃ ionic liquid. Continuouselectrodeposition of Al was not observed using other ionic liquids.

[0054] While the present invention has been described with reference tospecific embodiments, it is not confined to the specific details setforth, but includes various changes and modifications that may suggestthemselves to those skilled in the art, all falling with the scope ofthe invention as defined by the following claims.

1-24 (Cancelled).
 25. An electrolysis cell for refining or recycling a metallic element in an anode of the cell, the electrolysis cell comprising a cathode including a porous basket and electrically conductive particles held by the porous basket; an electrolyte including an ionic liquid containing a chloride of the metallic element; and a container holding the cathode and the electrolyte.
 26. The electrolysis cell according to claim 25, wherein the porous basket comprises a material selected from the group consisting of Al, Cu and stainless steel.
 27. The electrolysis cell according to claim 25, wherein the electrically conductive particles comprise an element selected from Al and C.
 28. The electrolysis cell according to claim 25, wherein the ionic liquid further comprises 1-butyl-3-methylimidazolium chloride.
 29. The electrolysis cell according to claim 25, wherein the chloride of the metallic element comprises AlCl₃.
 30. The electrolysis cell according to claim 25, wherein the chloride of the metallic element is a gas.
 31. The electrolysis cell according to claim 25, wherein the metallic element is selected from the group consisting of Li, Mg, Al, Ti, Zr and Nd.
 32. The electrolysis cell according to claim 25, wherein the porous basket is a mesh or perforated sheet and is sufficiently porous to permit electrolyte and dissolved material from the anode to circulate in the electrolysis cell.
 33. The electrolysis cell according to claim 25, wherein the porous basket comprises metal.
 34. The electrolysis cell according to claim 33, wherein the porous basket comprises Al. 